This invention relates generally to closed loop air/fuel ratio control of an internal combustion engine mounted on a motor vehicle or the like, and more particularly, the present invention relates to a method and apparatus for controlling the mixture of air and fuel supplied to internal combustion engines at a variable ratio in response to a signal derived from an exhaust gas sensor to reduce the emission of noxious components in burnt gases.
Various methods and systems for effecting air/fuel ratio control are known, and in one conventional method, a first integration corrective setting or correction factor is derived by integrating the output signal from the gas sensor, and then a second corrective setting or correction factor is derived in accordance with the first correction factor and the operating condition of the engine. The second correction factor is stored in a memory so that feedback control will be effected by determining the air/fuel ratio supplied to the engine by correcting or modifying a basic fuel flow amount, which is derived on the basis of the intake airflow and the engine speed, by the first and second correction factors. In such a known system, in which so called learning control or correction is effected, the second correction factor is apt to assume a value far deviated from its standard value due to rich mixture caused by fuel vapor supplied through the canister which collects evaporated fuel in the fuel tank.
If the engine is stopped under such condition, the data for the second correction factors remain in the memory. Therefore, when the engine is restarted after being cooled, the second correction factors, whose values are far deviated from their standard values, will be used to erroneously control the air/fuel ratio resulting in undesirable operation of the engine and emission of noxious components.